The present patent application is related to fiber optic networks, and, in particular, optical transmitters for WDM and DWDM network systems.
In WDM (Wavelength Division Multiplexing) fiber optic networks, optical signals are sent at predetermined wavelengths over optical fibers. Each predetermined wavelength forms a communication channel in the network and the wavelength (or frequency) of the optical signal is used to control the destination of the signal through the network. An advanced version of WDM networks is the DWDM (Dense Wavelength Division Multiplexing) network in which the number of wavelength channel is increased by reducing the channel wavelength separation. In DWDM networks, the communication channels are separated by 100 GHz, as set by the ITU (International Telecommunications Union) and the fiber optic industry is already assuming an unofficial channel separation of 50 GHz for advanced DWDM networks.
In passing, it should be noted that the term, WDM, is used to refer to both WDM and DWDM networks, unless indicated otherwise.
A challenge for WDM and DWDM networks is to ensure that the optical signals in one communication channel remain locked at that wavelength and much design effort has been spent in ensuring that the optical transmitters of a WDM system do not stray from their assigned channel wavelength. Heretofore, such efforts have required a channel wavelength reference element for each optical transmitter. Such wavelength reference elements are expensive and the number of channels in a typical WDM network system, eight or more, multiplies the costs for the element and the overall optical transmitter system. With the number of channels increasing and tighter wavelength channel separations, costs of conventional optical transmitter systems are expected to increase significantly.
The present invention is directed toward a wavelength stabilization system and method of operation which uses only one wavelength reference element. Hence the present invention provides for lower manufacturing costs for present WDM optical transmitters and in future systems as well.
The present invention provides for an optical transmission system for generating light signals at a plurality of predetermined wavelengths on an output fiber. The system has a plurality of modulated laser sources coupled to the output fiber and a feedback loop connected to the output fiber. Each laser source has an output for one of the predetermined wavelengths. The feedback loop includes a first subloop which generates electrical signals indicative of the total amount of light carried on the output fiber, and a second subloop which generates electrical signals indicative of the amount of light carried on the output fiber at the predetermined wavelengths. A control unit coupled to the plurality of modulated laser sources controls the output wavelength of each of the laser sources. The control unit receives the electrical signals from the first and second subloops and controls the laser source output wavelengths so that the output of each of the laser sources is centered at one of the predetermined wavelengths. A comb filter is in the second subloop for transmitting light signals at the predetermined wavelengths.
In another embodiment of the optical transmission system, the feedback loop includes a first filter tunable so as to controllably transmit light over the plurality of predetermined wavelengths; a first subloop generating electrical signals indicative of an amount of light transmitted by the first filter; and a second subloop generating electrical signals indicative of an amount of light transmitted by the first filter and and by a second filter having a transmission characteristics profile where the predetermined wavelengths fall upon the slopes of the transmission profile. The control unit coupled to the plurality of laser sources and to the first filter for tuning the first filter over the plurality of predetermined wavelengths receives the electrical signals from the first and second subloops and controls the laser source output wavelengths so that the output of each of the laser sources is centered at one of the predetermined wavelengths.
The present invention also provides for a method of generating light signals at a plurality of predetermined wavelengths on an output fiber in an optical transmission system. The system has a plurality of modulated laser sources each having an output for one of the predetermined wavelengths. The method includes tapping a portion of the light signals from the output fiber; dividing the portion into a first subportion and a second subportion; determining the amount of light in the first subportion; filtering the second subportion with the comb filter, determining an amount of light in the filtered second subportion; and controlling each of said laser sources so that the amount of power in the filtered second subportion is at a limit.
In another embodiment of the light signal generating method, the tapped portion from the output fiber is filtered to controllably transmit light over over the plurality of predetermined wavelengths and the filtered portion is divided into a first subportion and a second subportion. The amount of light in the first filtered subportion is determined. The second subportion is filtered with a comb filter which has transmission characteristics profile where the predetermined wavelengths fall upon the slopes of the transmission profile, the amount of light in the filtered second subportion is determined. The output wavelength of one of the laser sources is adjusted so that the amounts of light in the filtered first and second subportions are at a predetermined ratio to center the output of the one laser source at one of the predetermined wavelengths.